Boy, you'd think by now Apple could nail a launch, but it seems like they always have problems if it's something web-based (MobileMe, I'm looking at you). Case in point, my first attempt at a purchase in the Mac App Store:
Some nice touches, my purchased apps — even ones purchased outside the Mac App Store (contrary to what I've read on Ars) — show up as "Installed:"
And with the Mac App Store you can now buy portions of Apple's product suites — iLife and iWork — discretely, rather than only as a bundle:
Finally, I hate the iTunes shopping experience, and the Mac App Store is clearly based on it. I really hope someone comes up with a better way to shop. In the meantime, I'm just glad I don't have to actually open iTunes to shop the Mac App Store.
It's a start.
Ah! That's better! Looks like you can get this error to go away by just quitting the Mac App Store, opening iTunes, then reopening Mac App Store. This will let you agree to the End-User License Agreement, after which you can make purchases from the store.
On other oddity: Remote Desktop appears on the Mac App Store. But other downloadable apps, like Safari or iTunes, do not.